1. Field of the Invention
The present invention generally relates to the design of hulls for watercraft and, more particularly, to hull designs which are reconfigurable for alteration of operational characteristics thereof.
2. Description of the Prior Art
Waterborne vessels have been used since ancient times and for many purposes. The need for vessels to perform in a variety of water and weather conditions and the various purposes for which such vessels have been intended has led to a great variety of hull designs. In particular, the often conflicting requirements for operation in shallow water and under a variety of weather and water conditions and at high speed has led to the development of hydrofoil designs.
Hydrofoils function to increase speed by decreasing the wetted area of hulls by producing lift sufficient to support the main hull (hereafter sometimes referred to as a displacement hull for the purpose of indicating that it is a source of static lift for the vessel) of the vessel above the water and, in general, have been relatively successful for passenger service and the transporting of relatively light loads. However, the need to generate sufficient lift to entirely replace the function of the displacement hull as a means for supporting the vessel requires relatively high speeds which increases fuel consumption and reduces the range of the vessel. Further, the structure required to produce such lift increases with the displacement of the vessel and the size of vessels to which hydrofoils can be applied has, as a practical matter, been relatively limited. Such structures also increase the effective draft of the vessel, particularly in the hull-borne mode of operation and compromise the ability of such a vessel to operate in shallow water.
As an alternative for the purpose of extending the potential of hydrofoils to larger vessels, the so-called hybrid hydrofoil concept has recently received substantial interest. This type of design uses one or more submerged hulls or pods connected to the vessel by one or more struts as a structural base upon which hydrofoils can be mounted. The pod can be used to carry fuel and/or motive power systems and preferably provide some positive static buoyancy for the vessel. By providing a significant amount of support for the vessel by the static buoyancy of the pod, the hydrofoils are thus required to provide less dynamic lift, often only on the order of 30%-70% of the displacement of the entire vessel.
A particular design for a hybrid hydrofoil configuration has included a single pod with counter-rotating propellers (possibly co-axial) connected to the displacement hull of the vessel by a single narrow strut running a substantial portion of the length of the vessel. Two pair of foils, each equipped with flaps for producing lift and dynamic stabilization of the vessel are provided near the fore and aft ends of the pod. By the combination of providing a portion of the vessel support through the static buoyancy to the pod and merely using the foils to lift the displacement hull from the water, reducing wetted area of the combination hull of the vessel, the latitude of operating conditions has been increased.
Other designs have been used for particular purposes. In particular, so-called catamaran, trimaran and other multi-hull designs have been used effectively to decrease hull draft and effectively decrease the beam to provide increased lateral stability of the vessel. Catamaran and trimaran hull designs, while increasing wetted area, often provide increased speed since the beam of each of the plural hulls can be made smaller, presenting reduced total frontal area of the vessel. However, maneuverability is often reduced in such multi-hull designs.
General principles of multi-hull designs have also been proposed for implementation in hybrid hydrofoil designs by providing one or more pods for a vessel displacement hull. However, by the general nature of any hydrofoil design which must provide for lift of the vessel, the benefits of shallow draft of multi-hull designs is usually lost, particularly in the hull-borne mode of operation. Further, for applications which require the vessel to have a shallow draft, the hybrid hydrofoil, by utilizing the pods for propulsion, present a substantial risk of propeller damage or fouling.
Operation on shallow waterways such as lakes and rivers, portions of which may be only marginally navigable, are a typical application which requires a vessel to have a shallow draft. In some applications, it is preferable not only to minimize draft but also to provide for propulsion above the water surface such as with a ducted fan to avoid interaction with the bottom. In such cases, directional control is often provided by redirection of thrust such as with an air rudder. However, shallow draft reduces the stability and controllability of such a vessel. Shallow draft may also reduce the ability of the vessel to hold course during high wind and adverse sea conditions. Under such conditions it is preferable to have substantial wetted hull area.